Biomedical electrodes are a primary component of many medical devices including cardiac pacemakers and defibrillators, deep brain stimulation devices, cochlear implants, peripheral nerve (sacral, phrenic, vagal, etc.) stimulation devices, spinal cord stimulation devices for pain management, and diagnostic tools. The electrode(s) found on the tip of biomedical leads are placed in contact with the appropriate target tissue (brain, nerve, muscle, heart, cell culture, cell suspension, etc.) are used to transmit bio-electrical signals to and from the device and target tissue. The electrodes can be used to monitor or sense physiological activity, such as heartbeat, and to deliver therapeutic electrical pulses, such as pacing signals.
Many medical devices require both efficient electrical signal transduction and direct mechanical coupling with functional cells and tissue in order to maximize therapeutic benefits and reduce adverse effects. Unfortunately, many medical devices suffer from problems delivering or monitoring electrical activity due to lack of proximity from functional target cells or tissue. Implanted devices made from stiff materials including metals, ceramics, and plastics, produce tissue reactions that include formation of electrically-insulating scar tissue that mechanically separates the electrode from target tissue and impedes electrical conduction. In vitro diagnostic electrodes are often separated from target cells by layers of dead cells or adhered proteins that make electrical transfer slow and less effective.
Examples of implanted devices include cardiac pacemakers and cardiac resynchronization therapy which electrically monitor the heart's activity and then deliver electrical pulses to regulate cardiac contractions. Deep brain stimulators are used to monitor neural activity and deliver therapeutic pulses to prevent dyskinesia associated with Parkinson's disease, or to monitor and prevent seizure activity. Spinal cord stimulation involves the delivery of electrical pulses to electrodes implanted in or near the spinal cord in order to counteract chronic pain. Cochlear implants provide auditory sensation to persons with severe hearing loss by sending acoustic information in the form of electrical signals to an array of small metal electrodes implanted within the cochlear structure in the inner ear. Electrical signals sent to the vagus nerve, sacral nerve, and other targets in the peripheral nervous system are used to treat a number of diseases and disorders including but not limited to chronic pain, urinary incontinence, loss of diaphragm control, heart failure, and blood pressure. Implantable cardio-defibrillators monitor cardiac activity and upon observing heart failure deliver large electrical shocks to restore cardiac function.
Inherently conductive polymers (π-conjugated conductive polymers) and non-conductive polymers with conductive dopants are useful as biocompatible polymeric coating materials for preexisting electrodes, probes, and sensors providing unique electrical, biochemical and electroactive properties. The monomers that polymerize to form conductive polymers can comprise one or more of 3,4-ethylenedioxythiophene (EDOT), pyrrole, anilines, acetylenes, thiophenes, and blends and/or derivatives thereof.
Surface and bulk materials currently used as electrodes for biomedical devices offer limited biocompatibility, resulting in tissue injury and inflammation in the vicinity of the implanted device. In addition to limited biocompatibility, stimulation of chronic negative immune system reactions often lead to biofouling of existing implants of electrodes and erosion of device surface materials. Various biological tissues, including the central nervous system (CNS) react negatively to implanted devices, varying in severity according to the site of implantation, the materials used and differences in electrode geometries and implantation methodologies. Chronic rejection of the implantable devices in the CNS can be characterized by a hypertrophic reaction from surrounding astrocytes with increased expression of filament proteins and vimentin. In addition to protein adsorption to the device surface, microglial cells and foreign body giant cells envelop the implanted devices resulting in encapsulation of the device and formation of high electrical impedance fibrous scar tissue. This diminishes, and eventually negates signal transduction between the tissue and the device. Similar foreign-body responses are found throughout human and animal tissues including major targets for novel implanted biomedical devices including the brain, heart, and skin. Bio-incompatibility represents a key weakness of new implantable biomedical devices currently being developed and is the foremost roadblock to successful in vivo testing and usage.
Surface modification of implantable electrodes and sensors has been shown to provide improvements in both their long term biocompatibility and electro-functionality. It would be highly desirable to design electrode devices which could intimately interface electrode sites to living tissue, as well as to facilitate efficient charge transport from ionically conductive tissue to the electronically conductive electrode and induce surrounding tissue to attach or interface directly to the implanted device.
Hydrogels are materials formed from lightly crosslinked networks of natural or synthetic polymers such as saccharides that contain high water content, typically 90% (w/v) or more. The crosslinking of hydrogels can be achieved by various methods such as ionic, covalent chemical, or UV-initiated chemical. The method and degree of crosslinking affect the resorbability or permanence of the hydrogel. Hydrogels such as alginate, a material derived from brown seaweed, are currently used in FDA-approved medical applications for wound closure, embolism treatment, and drug delivery. The soft hydrogel materials provide a porous 3-dimensional matrix for cell ingrowth and also help interface stiff materials with tissue by buffering tissue from the device.